The Tennessee Valley Authority (TVA) undertook development of the process for the production of elemental phosphorus soon after the agency was created on May 18, 1933. Although elemental phosphorus had been produced commercially by smelting phosphate ore in blast furnaces and in electric furnaces, both production methods needed further development.
A hazardous waste problem arose immediately after TVA undertook the production of elemental phosphorus. Phosphorus vapor was condensed by contacting the furnace gases with water, and the liquid phosphorus must be kept submerged in water to prevent its spontaneous combustion upon contact with air. A characteristic of the condensation process is that part of the phosphorus condenses as small particles. The phosphorus particles became associated with water and small solid particles to form an emulsion called phosphorus sludge. Solid particles in the furnace gas may result from abrasion of the furnace feed materials (phosphate, coke and silica rock) and by volatilization of solids in the furnace followed by their condensation as small particles.
It was difficult to cope with phosphorus sludge. An attempt was made to separate elemental phosphorus from the water and solids by sedimentary methods, but such separation was not very successful.
Phosphorus sludge is not a stable emulsion and gravity causes a partial break down and separation of elemental phosphorus. Storage of phosphorus sludge in tanks will result in the separation of phosphorus as a layer in the bottom of the tanks. Phosphorus particles coalesce during storage to form the liquid layer in the tanks, and the phosphorus can be removed by pumping. Unfortunately the viscosity of phosphorus sludge increases as the phosphorus separates. When phosphorus sludge is first formed the elemental phosphorus content may be about 68.9 percent as reported in "Production of Elemental Phosphorus by the Electric-Furnace Method," Tennessee Valley Authority, Chemical Engineering Report No. 3, 1952, page 167. After extended storage in tanks the elemental phosphorus content was found to be in the range of 7 to 30 percent and the viscosity had become too high for the material to be pumped. When phosphorus sludge is burned to produce phosphoric acid it was found that the elemental phosphorus content had to be a minimum of about 60 percent. Oxidation of elemental phosphorus to P.sub.2 O.sub.5 is incomplete when the elemental phosphorus content is less than about 60 percent.
A 14-acre settling pond was provided to store accumulated phosphorus sludge at TVA. Also, phosphorus sludge was stored in sumps, tanks, railroad cars, and a landfill. Phosphorus sludge accumulated at other plants in ponds, sumps, and tanks.
One publication, "Recovery and Removal of Elemental Phosphorus from Electric Furnace Sludge,"I. J. Anazia, J. Jung and J. Hanna, Minerals and Metallurgical Processing, May 1992, reported that 5 to 10 percent of the phosphorus produced ends up in the sludge. Another reference, "The Recovery of Elemental Phosphorus from Phosphorus Bearing Sludges," Doctoral Thesis of William James Shaffner, The University of Western Ontario, 1975, states, "It has been reported by several workers that the phosphorus remaining in the sludges represents up to 3 to 5 percent of the total phosphorus produced." The first was probably for phosphorus furnaces operating without electrostatic precipitators to clean the furnace gas. The lower loss of 3 to 5 percent was for phosphorus furnaces operating with electrostatic precipitators.
After about 1939 phosphorus furnaces at TVA were equipped with electrostatic precipitators and loss of elemental phosphorus in phosphorus sludge was taken to be 4 percent. However, a waste was generated which was called precipitator dust. The precipitator dust contained elemental phosphorus that had absorbed on dust particles. About 20,000 tons of precipitator dust has accumulated at TVA.
All wastes that contain elemental phosphorus are called phosphorus-containing waste. This includes phosphorus sludge, phosphorus particles that have coalesced forming a layer under precipitator dust, and precipitator dust.
About 1.1 million tons of elemental phosphorus was produced over the approximately 40-year period the chemical was produced at TVA. From this it was estimated 45,800 tons of elemental phosphorus has accumulated in phosphorus-containing wastes at TVA.
The value of products that may be made from TVA's phosphorus-containing waste was estimated and the results are given in table 1.
TABLE I ______________________________________ Value of Products that Can Be Made from Phosphorus-Containing Waste at TVA Product Million $ ______________________________________ Elemental phosphorus from phosphorus sludge 114.6 Elemental phosphorus from smelting precipitator dust 6.9 Byproduct ferrophosphorus from smelting precipitator dust 0.1 Elemental phosphorus from smelting inorganic solids in 13.4 phosphorus sludge Byproduct ferrophosphorus from smelting inorganic solids 0.2 in phosphorus sludge Total 135.2 ______________________________________
It is evident innovative technology is needed to recover the wastes so as to improve the overall economy of phosphorus production. Although byproduct ferrophosphorus will be produced when inorganic solids are smelted, nearly all the value from the wastes will be from recovery of elemental phosphorus and from production of the chemical when the inorganic solids are smelted.